Generally, a straight ruler serves to measure a length of an object and a scale ruler serves to measure a reduced size of a real object. Generally, a scale ruler has a triangular cross section and serves to measure a length of an object which is identical to an original object and is reduced with a predetermined ratio, and the scale ruler shows a real length of an original object as the scale ruler measures the reduced object. Generally, the unit of a scale ruler is ranged from 1/100 m to 1/600 m.
However, prices of scale rulers are expensive than the original straight measure ruler. Furthermore the scale rulers have only six units so that it cannot be widely used in various situations. Therefore, a user must prepare several scale rulers for practical usages, but this is non-economic and inconvenient in storage and carrying. Furthermore the triangular cross section of a scale ruler occupies a larger space.